Interesting advice from everyone. When I worked at a store like that, it was ALL about the vibe. If I had shown up in normal interview gear I think they would have politely contained their laughter until I left and then passed on hiring me. I say dress to match the vibe. Besides knowing how to handle the snowboards, the whole skill set they generally look for in places like this involves fitting in with and attracting the kind of people who snowboard.
If you feel like there is a chance that somebody with hiring power is some kind of stickler for traditional business clothes (and I can't imagine anybody like that sticking around at a snowboard shop long enough to be important), then wear the chinos with something like this:
Then again, I've never been to the Denver area and there is way more snowboarding going on there than there is in any of the places I've lived, so maybe there are so many snowboard shops that there ARE some who expect you to look like you would at an office job. It just sort of strikes me as unlikely is all. Good luck!